The invention relates to a process for improving the signal-to-noise ratio in holography using bacteriorhodopsin-based recording media.
The application of bacteriorhodopsin or variants of bacteriorhodopsin as an active component in optical recording media is known. For example, Japanese application number JP 60/184246 A2, describes the recording, reading and transcription of data into thin bacteriorhodopsin films applied to a substrate. Also a general article by P. Kouyama et al., "Structure and Function of Bacteriorhodopsin", was published in Adv. Biophys. 24 (1988), pp. 123-175 and discusses the background of these possible practical applications.
In order to achieve greater stability in relation to thermal, chemical and photochemical decomposition or degradation, in optical recording media, preference is given to the use of bacteriorhodopsin and/or variants thereof as the active component. This is not in the form of free molecules, but in membrane-bound form, i.e., in the form of purple membrane or variants thereof, possibly comminuted.
The purple membrane can be obtained, for example, from Halobacterium halobium, and contains the photochromic protein bacteriorhodopsin. Variants of the purple membrane, i.e., those which contain variants of bacteriorhodopsin, may be obtained by known biotechnological processes. For example, variation of the retinal chromophores by means of chemical exchange or modification of the bacterio-opsin molecule are known processes (cf., for example, W. Gartner, D. Oesterhelt, 1988, Methoxyretinals in Bacteriorhodopsin. Absorption Maxima, Cis-trans Isomerization and Retinal Protein Interaction, Eur. J. Biochem. 176:641-648, or Soppa, J. et al., 1989, Bacteriorhodopsin Mutants of Halobacterium spec. GRB. 1 or 2, J. Biol. Chem. 264: 13043-13048 and 13049-13056, respectively). Examples of such appropriate variants are also described in this article. Equally suitable are pigment systems related to bacteriorhodopsin. Some of these are halorhodopsin or sensorrhodopsin, which, just like bacteriorhodopsin, may occur in free, crystalline or membrane-bound form, in wildtype form or with a modified chromophore or a modified amino-acid sequence.
For reasons of simplification, the expression "bacteriorhodopsin-based recording media" is used in the text to denote such appropriate recording media, which contain bacteriorhodopsin, halorhodopsin or sensorrhodopsin or variants or mutants thereof as the active component. However, the remarks made in this connection are also applicable mutatis mutandis to other appropriate pigment systems, without specific reference being made thereto.
A particularly promising application of such bacteriorhodopsin-based recording media is to be found in holography. This includes not only the purely holographic processes but also those optical processes which include one or more partial steps involving holography.
In holography, preference is given to the use of the bacteriorhodopsin-based recording medium in the form of films or gels. These are embedded in carrier materials such as, for example, polymers, applied to substrates such as glass plates or in some other form permitting a reproducible laminar arrangement. The resultant advantages afforded include a favorable absorption range, a high attainable resolution, a large number of possible write/erase cycles, a high degree of storage stability, and high sensitivity and light-fastness. Application in color holography is also possible. This applies to all hologram types which are possible using bacteriorhodopsin-based recording media. These embodiments are frequently designated as type B holograms, in which a photoconversion of the initial state (=B) of the active component is carried out, or those types of holograms in which an intermediate state or photointermediate is subjected to a photoconversion. Advantageously for this purpose, an intermediate is selected, the absorption properties of which are markedly different from those of the initial state. Such an intermediate which may usefully be employed for holographic purposes is available, for example, in the form of the intermediate state--frequently designated as the M state--of naturally occurring bacteriorhodopsin.
A serious disadvantage which has prevented more widespread application in holography of bacteriorhodopsin-based recording media such as those which contain purple membrane is the unfavorable signal-to-noise ratio. This can be observed when using conventional recording and reproduction processes. The reason for this is the predetermined, relatively low diffraction efficiency of the purple membrane. Also the large proportion of scattered light present, which proportion is influenced by such factors as the particle size of the purple membrane particles, factors determined by the production process, such as mechanical stresses and refractive index discontinuities with respect to the matrix material, and factors determined by the operating conditions, such as dust and fluctuations in temperature or humidity.